Today's PCs are many times more powerful than the relatively primitive PCs of the late 1970s and early 1980s. Current technology has produced a PC that essentially has the computing power of a minicomputer; it is a matter of time before PCs reach the power of mainframe computers. In tandem with the growth of the core PC has been a growth in number, complexity and variety of peripheral devices used in conjunction with PCs.
In the early days, a floppy disk drive might have been regarded as exotic, cassette tape players being more common. Today, in addition to floppy drives, PCs are likely to have a Winchester (also called a "hard" or "fixed") drive, a modem, a fax card, a printer, a tape backup unit, a compact disk-read-only memory ("CD-ROM") device, a sound card or, via a small computer systems interface ("SCSI") or music instrument digital interface ("MIDI"), other types of peripherals.
Unfortunately, as the number and complexity of peripherals grow, the chances of a fault occurring in the core PC or one of the peripherals escalate. Thus, it has become evermore important to test the PCs to ensure that they are completely functional before they leave the factory. Currently, such testing comprises the execution of a suite of diagnostic routines on each PC, the diagnostic routines designed to test each component of the PC thoroughly, and under a variety of operating conditions. To supplement diagnostic testing, PCs are often subjected to a lengthy "burn-in" period, during which time most component failures occur, if they are ever to occur.
One of the components that is the subject of diagnostic testing is a parallel (or printer) port of the PC. As will be described in more detail, parallel ports are standardized devices typically employed in a PC to allow the PC to drive a printer. Parallel ports consist of a single group of data lines and a number of control lines arranged in complementary pairs (one providing a status or query signal from the PC and another along which an associated answer or echo signal is transmitted back to the PC). Standard parallel ports have 8 data lines. Parallel ports in early PCs were unidirectional output ports, that is, they were capable of transmitting data from the PC, and not generally capable of receiving data into the PC. Such unidirectional ports are referred to as AT-style ports.
More recently, most PC parallel ports have become bidirectional (referred to as PS/2 style ports), allowing both output and input of data. Bidirectionality greatly increases the flexibility of parallel ports, thus increasing the overall power of the PC. Thus, it is vital to ensure the proper operation of the parallel port.
In the past, parallel ports were tested by being coupled to a loopback plug. A loopback plug, in the traditional sense, was a passive device (containing no signal processing circuitry) that merely coupled the complementary control line-pairs together and coupled one of the data lines to a complementary error line. Then, software executing within the PC caused a signal to be sent along each one of the control line-pairs and the data line. The loopback plug routed each of the signals sent back through the other one of the line-pairs (including the error line). If the signals came back intact, the port was deemed to be fully operative.
Unfortunately, such was not necessarily the case. As mentioned, one of the data lines was looped back through the error line. However, the other 7 data lines had no corresponding line to which to be looped back. (Recall that the data lines must be assumed to be unidirectional, as many parallel ports still are.) Thus, the remaining 7 data lines are untested and potentially faulty.
Accordingly, what is needed in the art is a test apparatus for testing the data lines in a parallel port, regardless of whether the parallel port is unidirectional or bidirectional. The test apparatus should preferably be small, inexpensive and simple to couple to and decouple from the port to allow rapid and accurate testing thereof.